Tuning arrangement



March 17, 1942. F. KREIENFELD TUNING" ARRANGEMENT Filed NOV. 10, 1939 T01'. F- CIRCUITS INVENTOR FRIEDRICH KRE/ENFELD ATTORNEY Patented Mar. 17,1942 TUNING ARRANGEMENT Friedrich Kreienfeld, Berlin, Germany, assignorto Telefunken Gesellschaft fiir Drahtlosc Telegraphic m. b. H., Berlin,Germany, a corporation of Germany Application November 10, 1939, SerialNo. 303,715 In Germany October 5, 1938 1 Claim.

It is known that oscillatory circuits of receiver apparatus may be tunedby the shifting of dust cores of the coils contained in the oscillatorycircuits. This method is used not only for the steady and continuoustuning in lieu of a rotary condenser, but it is also used in connectionwith oscillation circuits which are cut into circuit in automaticallytuned receivers by actuation of push buttons. These oscillatorycircuits, to be sure, are pre-tuned, but they are often tunable inside alarger range or frequency band in order that for each push button anydesired transmitter station of the wave band may be selected.

In order that synchronism of several tuning circuits throughout thewhole frequency band may be more readily realizable it is known to bedesirable that the frequency curve which gives the inter-dependencebetween the frequency and the shifting of the core should be as straightas feasible. For all that is then necessary is to take care that thefrequency characteristics of the Various circuits will present the sameslope, for by parallel shifting they can be made to register orcoincide, and this is not feasible when the curves are not linear. Inthe case of capacitive tuning with straight line frequency condensersequal slopes are obtainable by balancing the inductances and parallelshifting by mutual turning of the rotors. Relative departures ofdifferences of the inductance values, as will be seen, have the resultthat the frequency characteristics diverge as the frequency growsbecause the frequency difference of the two circuits, in the presence ofa difference in the inductance is not constant throughout the band. Infact, it is only the percentage frequency difference that is constant(because of the constant percentage inductance difference) and theresult is that also the frequency difference is correspondingly higheras the frequency increases.

Identical considerations apply in an analogous manner to inductancetuning which is here of interest. In this case, synchronism is securablefor a straight line frequency tuning characteristic by causing the slopeof the characteristics to coincide or register by balancing the capacitywhich remains unaltered during the tuning, and by causing thecharacteristics to coincide by mutual shifting of the interlocked dustcores. The

advantages inherent in this method are as follows: No additional circuitelement is required to the end of balancing or trimming the initialinductances, in fact, relative shifting of the dust cores suffices.Moreover, because of the absence of such an additional or accessorycircuit element for balancing the initial inductances, the range ofvariation of the tuning is not restricted.

For a better understanding of the invention reference will be had to theaccompanying drawing wherein, Fig. 1 represents curves depicting therelation between frequency and core displacement which will serve toexplain the invention, Fig. 2 shows a coil arrangement according to theinvention for obtaining increased linearity between core displacementand the frequency of the tuned circuit, and Fig. 3 shows the applicationof the invention to a circuit of the superheterodyne type.

The invention is predicated upon the fact that the curve which gives theinter-dependence between the frequency of the oscillatory circuit andthe shift of the dust core has a form similar to that shown by the solidgraph in Fig. 1. Along the rectilinear portion of the graph thedependence of the frequency upon the shifting of the dust cores followsa straight line law. The straight portion in Fig. 1, however, is toolimited, in fact, it is desirable to extend this range. This is attainedby the invention.

According to the invention the cylindrical coils, comprised in theoscillatory or tuning circuits of a receiver apparatus in which thetunings are interlocked by shifting dust cores, are provided, at theirends which are reached last by the cores as they are shifted in, withadditional layers of turns over a distance so that there will be anincrease in the tuning range through which a straight line frequency lawis followed.

Referring to the drawing, Fig. 1 shows the interdependence between theshift distance or length l of a dust core and the frequency f of theoscillatory circuit. The beginning of the graph below on the left handside shows the frequency of the tuning circuit when the dust core isremoved. As the core is approached, the frequency initially drops slowlyand later at a growing rate. Now, this drop follows a straight line lawinside a limited range. At the upper end, the curve bends over. By theinvention the top bend is shifted further out, at the same time itbecomes sharper, with the result that the rectilinear portion of thecurve is extended.

Fig. 2 shows how this end is attained according to the invention. Thereis first wrapped upon the form 4, a coil l which comprises a singlelayer of spires or turns. Over a portion equal to about one-third of thelength of the coil, one or several additional layers of turns 2 arewrapped. Thus, as the core 3 is inserted the inductance grows morerapidly as the iron core at the end of the coil comes to act upon agreater number of turns. Inside the portion of the graph shown in Fig.l, as will be noticed, the inductance of the coil grows in accordancewith a square law.

The invention is not only useful in connection with equally tunedcircuits, but it will be found suitable also in superheterodynereceivers in the input circuit and the oscillator circuit. Thedifference in frequency between the two circuits which is equal to theI. F. may be obtained in various ways. For instance, the two interlockedcores may be so shifted relative to each other that at one end of therange or band the desired frequency dilference is obtained. Then,provided that the capacities are correctly adjusted it will prevailthroughout the entire range. However, such shifting of the dust cores ispracticable only when the I. F. is low, for otherwise the range ofvariation would be unduly restricted. In order to avoid this, dust coresof dissimilar permeability may be employed, or else one of the coils maybe wound with greater pitch or be made of a larger diameter. But it isalso possible to use like coils and cores for both circuits, but then acoil should be connected in parallel and another, small coil beconnected in series relative to the coil contained in the circuitoscillating at the higher frequency, in line with what is done in caseof capacitive tuning where a parallel and a series only a singleparallel inductance and a single series inductance need be provided,seeing that the oscillatory circuits are cut in circuit alternately.Such an arrangement is shown in Fig. 3 of the drawing.

In said figure the invention is shown incorporated in a receiver of thesuperheterodyne type which employs a combined oscillator and first detector tube l3. Signals from the antenna circuit H), H are impressedupon the third grid of said tube and used to modulate, so to speak, theoscillations generated by the portion of the tube comprising thecathode, the first grid and the second grid. With such an arrangementthe plate circuit contains the products of the combined localoscillations and the received energies from which the so-calledintermediate frequency is selected. The signal input circuit of tube I3is tuned by means of the condenser l2 and whichever one of the coils I9,21, etc. happens to be shunted across the condenser l2 by the switch 34.In other words, if the switch 34 is in position I the inductance coil I9is connected across the condenser l2 and the two form a tuned inputcircuit for the tube l3. However, if switch 34 is in position 2 theinductance coil 21 is shunted across condenser l2 and forms with thecondenser a tuned signal input circuit for the tube [3. Only two of theinductance coils have been shown in Fig. 3 since it is obvious that anynumber thereof may be provided and suitably connected to the positions3, 4, 5 etc. of the switch 34. The frequency determining circuit for thelocal oscillator portion of the tube l3 includes the condenser l5 andwhichever one of the inductance coils 20, 28, etc. happens to beconnected across condenser by switch 35. Here again only two coils havebeen shown. However, it is to be understood that any number of coils maybe provided depending upon the number of stations desired to be tuned inautomatically and the number of oscillator coils will correspond to thenumber of coils employed in the input circuit.

The switches 34 and 35 may be uni-controlled by means 36 shownschematically. The single parallel inductance l6 and single seriesinductance I! are associated with the oscillator circuit and these twoinductances cooperate with whichever one of the inductance coils 2B, 28etc. happens to be in the circuit. Coils I9 and are provided with cores2| and 23 respectively which may be of the powdered iron core type.These two cores are connected by a suitable piece of insulation material22 and are uni-controlled by means of the screw thread 24 cooperatingwith the plate 26 to facilitate adjusting the cores 2| and 23 withintheir respective coils by operation of screw head 25. By turning thescrew head in one direction the cores enter into their respective coilswhereas turning the screw head in the opposite direction withdraws thecores. The same sort of arrangement is used for the two coils 27 and 28which are associated with the cores 29 and SI respectively. To set thedevice so that the first position of switches 34 and 35 tunes to aparticular station all that need be done is to place switches 34 and 35into the first position by operating the uni-control means 36 and thenmanipulating screw 24, 25 until that station is tuned in. Then theswitch 34, 35 is placed into its second position and screw 32, 33operated so as to tune to some other desired station and so on for allthe various circuits provided. Thereafter, all that need be done to tuneto any desired station which has been previously set to is operate thedevice 35 so as to move switches 34, 35 to the desired position. It is,of course, to be understood that the coils I9, 20, 21 and 28 may beconstructed like the coil shown in Fig. 2 in which case the inductancesl6 and H are not absolutely necessary.

I claim:

In a resonant circuit, a variable tuning instrumentality comprising aninductance winding provided with a core of substantially the same lengthas said winding, said core being adapted to be substantially completelyinserted into said 4"- winding from one end thereof, means for extendingthe rectilinear portion of the characteristic curve of said resonantcircuit relating the resonant frequency of the circuit to thedisplacement of the movable core within the winding comprising anauxiliary winding connected in series with said first named winding, theturns of both said windings being in the same direction, said aux iliarywinding being superimposed upon a portion of the winding which is in thevicinity of the 1 other end thereof.

FRIEDRICH KREIENFELD.

